Recently, decahedral titanium oxide particles containing mainly anatase-type crystals (below, abbreviated as “decahedral titanium oxide particles”) and a production method therefor have been reported (Patent Documents Nos. 1 and 2, and Non-Patent Document No. 1). In addition, it is also reported in these documents that the decahedral titanium oxide particles have high activity as a photocatalyst.
The production method for the decahedral titanium oxide particles reported in these documents is a method in which a gas containing titanium tetrachloride and oxygen is rapidly heated and then rapidly cooled under certain conditions. However, the decahedral titanium oxide particles obtained by the production method have almost a particle diameter of 100 nm or greater. Therefore, it is difficult to selectively obtain decahedral titanium oxide particles having a particle diameter of 100 nm or less by these conventional production methods. There is a problem to be solved in which the particle size is decreased while maintaining a decahedral box-shape.
At the same time, as a method for producing fine titanium oxide particles, a method, in which oxygen and water vapor are used as an oxidizing gas when titanium tetrachloride is oxidized in vapor, has been suggested (Patent Document No. 3).    Patent Document No. 1: PCT International Publication No. WO 04/063431 brochure Patent Document No. 2: Japanese Unexamined Patent Application, First Publication No. 2006-52099    Patent Document No. 3: Japanese Patent Publication No. 3656355    Non-Patent Document No. 1: Kusano•Terada•Abe•Ohtani, 98th Catalyst Discussion Session (September, H18), Discussion Session Proceedings A, page 234